In digital transmission systems, distortions occur in transmission signals because of various factors such as signal transmission channel distortions, signal reflections, and changes in transmission channel characteristics in mobile radio communications or broadcast receiving systems. For this reason, it is required for receiving apparatuses to compensate distortions in transmission signals by a distortion compensation technique using waveform equalization, a transmission channel estimation technique for estimating states of transmission channels, an error correction technique and other techniques, to reproduce desired signals.
In order to perform synchronous reproduction and waveform equalization of reception signals stably or efficiently in receiving apparatuses, transmission systems for transmitting signals to which predetermined known signals are inserted have been practically used in the fields of broadcasting and communication. If these known signals have longer signal sequence lengths, synchronous reproduction, waveform equalization, transmission channel estimation and the like can be realized with higher accuracy in receiving apparatuses; but transmission speeds relatively decrease in transmission of information which is desired to be transmitted. For this reason, it is required to achieve transmitting/receiving systems capable of performing stable transmission/reception while desired information transmission speeds are secured.
Generally, in waveform equalizers that use a known signal as a teacher signal and CIR detectors that estimate a CIR (Channel Impulse Response) in a transmission channel from a known signal, digital filters such as transversal filters or decision feedback equalizers are used. Generally, in the waveform equalizers, a known signal is used as a teacher signal for making outputs obtained by filtering a reception signal closer to the known signal. As for the CIR detectors, a known signal is generally used as an input signal to filters. Filter coefficients are successively updated by an adaptive algorithm such as an LMS (Least Mean Square) algorithm. There is a problem that if a known signal sequence length is short, a filter coefficient cannot converge to an optimum value and a waveform equalizer or a CIR detector cannot obtain a desired performance.
To cope with such a problem, a method for repeatedly using a same known signal sequence to increase a known signal sequence length in appearance is proposed (see Patent Document 1, for example).